Facebook has reportedly admitted tracking which sites its users visit even after they log off, thanks to plug-ins and cookies.

Facebook, which has more than 800 million active users, also keeps close track of where millions of non-members of the social networking site go on the web, even after they visit a webpage for any reason only once, the 'USA Today' reported.

To do this, the company relies on tracking cookie technologies similar to the controversial systems used by Google, Adobe, Microsoft, Yahoo and others in the online advertising industry, Arturo Bejar, Facebook's Engineering Director, was quoted as saying.

Here's how it works: Every time one logs onto Facebook it inserts a "session cookie" and a "browser cookie" into one's browser. If one simply visits the site without signing up on the browser, cookie is inserted.

From that point on, each time one visits a site which uses Facebook technology, the cookie works in conjunction with the plug-in to alert Facebook to the date, time and URL of the page you are viewing.

The unique characteristics such as one's IP address, screen resolution, operating system and browser version, are also recorded by the social networking site, the report said.

Facebook spokesman Andrew Noyes said that the company's tracking systems are used to personalise content and help boost security, adding the tracking practices are spelt out in its 'Privacy Policy' and 'Help Center' web pages.

However, industry critics have expressed serious concern about the practice. "Tracking data can be used to figure out your political bent, religious beliefs, sexuality preferences, health issues or the fact that you're looking for a new job," Peter Eckersley of Electronic Frontier Foundation, a digital rights organisation, told the newspaper.